[en] The incidence of micronucleus formation was studied at 12, 24 and 36 h post-irradiation in the polychromatic (PCE) and normochromatic (NCE) erythrocytes of the bone marrow of mice whole-body exposed to 0, 3, 9, 18, 36, 54 and 72 cGy of ⁶⁰Co γ-radiation. It was observed that the frequency of micronucleated polychromatic erythrocytes (MPCE) increased with the increase in exposure dose at all the post-irradiation time periods studied. Similarly, the frequency of micronucleated normochromatic erythrocytes (MNCE) also increased with the increase in exposure dose and the increase for both MPCE and MNCE was dose related. The dose-response relationship was linear-quadratic for both MPCE and MNCE. The study of mitotic index revealed that a dose as low as 9 cGy is capable of reducing the mitotic index significantly at 24 h post-irradiation and the dose response was linear-quadratic. However, no significant decline in the mitotic index was observed at 12 and 36 h post-irradiation